Solve for $z$ : $-21 = -27 + z$
Add $27$ to both sides: $ \begin{eqnarray} \\ \hphantom{-21 {+ 27}} &\hphantom{=}& \hphantom{z} \\ -21 &=& -27 + z \\ \\ {+27} && {+27} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -21 {+ 27} &=& z \\ \end{eqnarray} $ Simplify. $ z = 6$